Attention has been focused on a technique for forming a transistor including a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor including an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
An oxide semiconductor film can be formed by a technique for forming a thin film, such as a sputtering method. Further, the oxide semiconductor film can be formed at a relatively low temperature compared to a silicon semiconductor or the like. Hence, the oxide semiconductor film can be formed to overlap with another transistor. For example, Patent Document 2 discloses a semiconductor device in which a cell area is reduced by providing, over a transistor including silicon, a transistor including an oxide semiconductor layer serving as a channel formation region.